1. Field
The present invention relates to a lithographic apparatus and a method for manufacturing a device.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs), micro-electro-mechanical-systems (MEMS), and other devices involving fine structures. In a conventional apparatus, a contrast device or a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display or other device. This pattern can be transferred onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a glass plate). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can comprise a patterning array that comprises an array of individually controllable elements. Compared to mask-based system, the pattern can be changed more quickly and for less cost.
In general, a flat panel display substrate is rectangular in shape. Known lithographic apparatus designed to expose a substrate of this type typically provide an exposure region, which covers a full width of the rectangular substrate, or which covers a portion of the width (e.g., about half of the width). The substrate is scanned underneath the exposure region, while the mask or reticle is synchronously scanned through the beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate, then exposure is completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate is moved transversely after the first scan, and a second scan is performed to expose the remainder of the substrate.
Another way of imaging includes pixel grid imaging, in which a pattern is realized by successive exposure of spots.
Where the pattern on the substrate is built up from a grid of localized exposures or “spot exposures,” it is found that the quality of the pattern formed at a particular point can depend on where that point is located relative to the spot exposure grid positions. Furthermore, a variation in pattern quality can be found to exist with respect to the angle of a feature in the pattern relative to axes defining the grid. Either or both of these variations can have a negative influence on the quality of a device to be manufactured.
The image log slope of a pattern determines the resist side-wall angle of features formed after processing of an exposed substrate. A shallow image log slope implies a shallow side wall angle, which can be useful, for example, for achieving a wide viewing angle for Flat Panel Displays or can reduce the consequences of overlay errors. Steeper image log slopes and side wall angles provide greater contrast. The maximal image log slope is determined by the point spread functions of the spot exposures in the grid, and on the geometrical properties of the grid. In general, therefore, the image log slope is fixed once the corresponding hardware elements have been finalized. However, it can be desirable to vary the image log slope according to the nature of the application.
The critical dimension (CD) refers to the size of the smallest printable feature. Although the CD of the dose pattern can be defined quite accurately prior to exposure, it is more difficult to predict the CD properties of the pattern after post-exposure processing. Frequently, it is desirable to tweak the CD after inspection of a processed substrate in order to optimize the processed pattern according to a customer's requirements. One way this can be achieved is to vary the intensity of the radiation source. The more intense it is, the more the resulting pattern is spread out (normally leading to an increased CD). However, CD biasing in this way can only be applied uniformly and in a circularly symmetrical fashion over the surface of the substrate.
Variation in the position of the substrate surface relative to the plane of best focus can cause deterioration in the quality of the image formed on the substrate. Complex servo and control systems can be provided to translate and/or tilt the substrate table and/or projection system in order to keep the substrate near the plane of best focus but it is difficult to achieve perfect compensation. A residual focus error tends to be remain.
Where an array of individually controllable elements is used as a patterning device, some form of conversion tool is to translate requested spot exposure doses to voltages suitable for actuating the corresponding elements of the array at the appropriate times. For example, where the array of individually controllable elements comprises a mirror array, the voltages will be chosen so as to cause individual mirrors or groups of mirrors to tilt in such a way as to deflect an appropriate portion of incident radiation through the projection system. The relationship between the proportion of deflected radiation and the voltage/tilt angle can be complex (e.g., non-linear). Factors that affect the intensity/uniformity of the radiation incident on the array of individually controllable elements and variations in the optical properties of projection system components (e.g., variations between different optical columns) can also affect the intensity of radiation reaching the substrate and thereby reduce the quality of the pattern formed.
Where an array of individually controllable elements is used as a patterning device, ghost light (i.e., light originating from elements other than those that are supposed to be contributing to a particular sub-beam of radiation) can cause errors in the pattern formed on the substrate.
Therefore, what is needed is a system and method that more efficiently and effectively performs maskless lithography.